* Sachs first showed (’Lehrbuch,’ etc., 4th edit. p. 452) the intimate connection between turgescence and growth.  For De Vries’ interesting essay, ‘Wachsthumskrümmungen mehrzelliger Organe,’ see ‘Bot.  Zeitung,’ Dec. 19, 1879, p. 830.

** ‘Die Periodischen Bewegungen der Blattorgane,’ 1875.

*** ‘Untersuchungen über den Heliotropismus,’ Sitzb. der K. Akad. der Wissenschaft. (Vienna), Jan. 1880.

[page 3] creased growth, first on one side and then on another, is a secondary effect, and that the increased turgescence of the cells, together with the extensibility of their walls, is the primary cause of the movement of circumnutation.*

In the course of the present volume it will be shown that apparently every growing part of every plant is continually circumnutating, though often on a small scale.  Even the stems of seedlings before they have broken through the ground, as well as their buried radicles, circumnutate, as far as the pressure of the surrounding earth permits.  In this universally present movement we have the basis or groundwork for the acquirement, according to the requirements of the plant, of the most diversified movements.  Thus, the great sweeps made by the stems of twining plants, and by the tendrils of other climbers, result from a mere increase in the amplitude of the ordinary movement of circumnutation.  The position which young leaves and other organs ultimately assume is acquired by the circumnutating movement being increased in some one direction. the leaves of various plants are said to sleep at night, and it will be seen that their blades then assume a vertical position through modified circumnutation, in order to protect their upper surfaces from being chilled through radiation.  The movements of various organs to the light, which are so general throughout the vegetable kingdom, and occasionally from the light, or transversely with respect to it, are all modified

* See Mr. Vines’ excellent discussion (’Arbeiten des Bot.  Instituts in Würzburg,’ B. II. pp. 142, 143, 1878) on this intricate subject.  Hofmeister’s observations (’Jahreschrifte des Vereins für Vaterl.  Naturkunde in Würtemberg,’ 1874, p. 211) on the curious movements of Spirogyra, a plant consisting of a single row of cells, are valuable in relation to this subject.

[page 4] forms of circumnutation; as again are the equally prevalent movements of stems, etc., towards the zenith, and of roots towards the centre of the earth.  In accordance with these conclusions, a considerable difficulty in the way of evolution is in part removed, for it might have been asked, how did all these diversified movements for the most different purposes first arise?  As the case stands, we know that there is always movement in progress, and its amplitude, or direction, or both, have only to be modified for the good of the plant in relation with internal or external stimuli.